Aluminum brazing

ABSTRACT

This invention is directed to a process for the fluxless vacuum brazing of aluminum articles in which successful brazing is accomplished without the necessity for complete degreasing which has heretofor been considered absolutely necessary.

United States Patent 1 1 Swaney, Jr.

[ ALUMINUM BRAZING [75] Inventor: 0. William Swaney, Jr., Richmond,

Ind.

73 Assignee: Philco-Ford Corporation,

Philadelphia, Pa.

22 Filed: Mar. 9, 1972 [21] Appl. No.: 233,283

521 u.s.cl .f. 29/494, 29/488, 29/497 511 mu ..B23k 31/02,B23k 35/38 58Field of Search 29/488, 494, 497

[56] References Cited UNITED STATES PATENTS 3,457,630 I 7/1969 Schwartzet al. 29/494 [111 3,747,199 [451 July 24, 1973 3,482,305 12/1969 Dockuset a1 29/494 X 3,497,944 3/ 1970 Antle 29/494 3,514,842 6/1970 Beuyukianet al. 29/494 Primary ExaminerRobert D. Baldwin AssistantExaminer-Ronald .1. Shore Attorney-Robert D. Sanborn [57] ABSTRACT 4Claims, No Drawings ALUMINUM BRAZING THE PERTINENT PAST It has long beenknown to braze aluminum articles with flux either at atmosphericpressure or under vacuum. The usual aluminum stock employed in theseprocesses has had a cladding of a layer of an aluminum alloy containingan element capable of depressing its melting point. The element usuallyused was silicon.

The brazing operation is conducted by subjecting the preformed articleusually in a supporting fixture to a temperature intermediate themelting point of the surface cladding alloy and the base metal which isessentially pure aluminum or aluminum with a minimum of alloyingadditions.

A process has recently been introduced to the art in which the claddingon the aluminum base destined for vacuum fluxless brazing has beenmodified by incorporating therein a small amount of magnesium oralternatively magnesium is caused to be present in the brazing chamber.This concept is treated in detail in the following United States LettersPatent Nos.:

THE INVENTION It has long been the belief in the aluminum brazingindustry that absolute chemical cleanliness of the surface is a sine quanon for satisfactory wetting and brazing. This invention is predicatedupon my discovery that such cleanliness is not only not necessary, butmaybe an unnecessary and harmful impediment to the entire process.

The fabrication of a typical aluminum brazed heat exchanger is anexample of this invention. Specific reference is made to theconventional plate and fin heat exchanger employed as an'evaporator in ausual automotive aircon'ditioning system. The clad aluminum sheets fromwhich this evaporator is fabricated are formed cold mechanically both incontinuo'us rolls and in conventional forming presses.

These rolling and pressing operations require the presence of alubricant to prevent undue wear. on the dies and rolls and tearing andmalforming of the aluminum parts. A very large variety of lubricants areavailable and satisfactory for this operation provided they arequantitatively removed prior to the actual brazing step. Thisquantitative removal is ordinarily accomplished by the so called vaporphase degreasing process.

This invention involves preheating the assembled and clamped aluminumarticle to be brazed to a temperature of 300 to 500F and subjecting theheated article to a vacuum of about 100 microns.

In a batch process this may be accomplished by introducing the aluminumarticles into the brazing furnace from atmosphere and then increasingthe vacuum and temperature, care being taken not to increase thetemperature sufficiently rapidly to crack or carbonize the lubricant.Many lubricants are available which will volatilize quantitatively atbetween 300 and 500F and 100 microns pressure. However such oils wouldtend to crack if heated to 500F at atmospheric pressure and the aluminumwould certainly oxidize. It is essential that the walls of the furnacebe kept within the specified temperature range so that the evaporatedlubricants will not condense on the walls, but will be drawn into thevacuum system and so disposed of.

It will be readily appreciated by those skilled in the art that thisinvention is just as applicable to continuous or semicontinuous processas to a batch process. In these non-batch processes the aluminumarticles are first placed into an entrance chamber which is capable ofbeing heated and evacuated and which is separated from the brazingchamber by a vacuum tight door. In these non-batch processes thevolatilization of the lubricant can readily be accomplished in theentrance chamber and the perfectly clean aluminum articles thentransferred to the brazing chamber.

It is further to be noted that the preheating of the aluminum articlesneed not be entirely accomplished in chambers capable of evacuation. Thepreliminary part of the preheating may be accomplished in the open airpreferably by radiant heat. Care must be taken not to crack thelubricant nor to oxidize the aluminum article in such atmosphericpreheating steps.

The lubricant used in the rolling and forming operations is a carefullyrefined and ash-free petroleum base composition known to the trade asCindol 3,401. However, any. lubricant having a similar quantitativevolatility may be substituted and successfully brazing accomplished.

After the lubricants have been satisfactorily volatilized, both thetemperature and vacuum are increased to effectuate the braze. Themaximum temperature reached is ordinarily in the vicinity of l,l60F andthe vacuum should be at least as high as 0.01 micron although under someconditions a vacuum of 0.1 micron will suffice. Again it is essentialthat the elevation of the temperature and the increase in the vacuum bescheduled so that at no time is the aluminum article exposed to acombination of temperature and exposure to oxygen or water vapor whichwill oxidize the exposed aluminum surfaces and inhibit proper wettingand brazing.

It has been found that brazed aluminum air conditioning evaporatorsproduced in accordance with the practice taught supra have a surfacecolor directly comparable to new aluminum as received from suppliers.The evaporators produced employing conventional degreasing procedureshave a distinctly darker or smoked appearance.

I claim as my invention:

1. The process of producing a bright surfaced aluminum article which hasbeen assembled from a plurality of closely fitted aluminum parts atleast a portion of which have been fabricated from aluminum sheet basestock clad with an alloy of aluminum having a melting point lower thanthe aluminum sheet base stock comprising fabricating such individualparts with the aid of a volatile lubricant, assembling the individualparts to form the final aluminum article, fixturing the individual partsin place and without the interposition of any quantitativelubricant-removal process, subjecting the fixtured assembly carryingresidual lubricant to vacuum and radiant heat which combined willvolatilize any residual lubricant remaining on the fixtured assemblywithout decomposition and remove such vaporized lubricants from thevicinity of the fixtured assembly, and then increasing the vacuum andradiated heat applied to the fixtured article to effectively completethe brazing operation.

2. The process recited in claim 1 in which at least a portion of theheating of the article for lubricant removal is accomplished prior tothe application of the vacuum.

3. The process recited in claim 1 in which the fix- 5 tured articles areheated to a temperature in the range

2. The process recited in claim 1 in which at least a portion of theheating of the article for lubricant removal is accomplished prior tothe application of the vacuum.
 3. The process recited in claim 1 inwhich the fixtured articles are heated to a temperature in the range of300* to 500*F for lubricant removal.
 4. The process recited in claim 1in which the vacuum employed for lubricant removal is about 100 microns.